Wel'll just have to disagree. If you have unlimited time and money, I agree that it's a good idea to go straight to a specialist. You can guess which one to start with-- I'm sure any specialist will be better at sending you to another specalist than a trainer would. And of course, you won't delay getting treatment because all specialists are available 24/7 for appointments without referrals.
If on the other hand, you are one of those rare people with limited time or money, or you encounter one of those rare specialists who can't see you on a moment's notice, you might want to start with a general practitioner or trainer.
As for trainers, you are right, I polled all of them in the US, and none of them have any medical knowledge whatsoever.
As you may see, I am being sarcastic. Let me give you an example of when you might want to consult a trainer. You are doing an exercise, and you hear a pop followed by intense pain in your lower leg. It's so bad you have to lie down. You ask your trainer what to do. He looks at your leg and says "the bone is sticking out, I think I should call 911." I haven't met a trainer who would not be capable of that differential diagnosis, and I don't think it's fair to trainers to assume that they can be no help at all in desiding how to procede when you have an injury. They may not be doctors (some are), but they can offer you advice on which doctor to see first, which is better than no advice.
Furthermore, even with doctors, with anything serious, it's a good idea to get a second opinion, and having opinions from a trainer, a GP, and a specialist is not a bad thing. A trainer mught know your form, and be able to suggest HOW you got injured better than a doctor. A GP will generally know your history better, and might see related conditions (such as that your cholosterol medicine is affecting your muscles). A specialist will be better at diagnosing a specific problem. But it's not my opinion that ruling out advice from others and going straight to specialists for everything is a good practice. I think everyone has something positive to offer, and you should use all of their services appropriately.